Thrusters or rocket engines for propulsion have been in use since antiquity for warfare. Such early rockets used solid propellant materials, notably slow-burning varieties of gunpowder. Gunpowder, however, is an explosive, rather than a propellant, and has proven to be unreliable when used for propelling rockets, because of the likelihood of explosion.
More modern solid propellants reduce the likelihood of explosion, but still suffer from the disadvantage that, once ignited, ignition tends to continue until the propellant is exhausted. Also, the thrust generation of a solid-propellant rocket depends, from moment to moment, upon the configuration of the partially-burned propellant; a propellant which is in the form of a solid block will burn at its surface, and produce less mass flow per unit time (W) of combustion or reaction products, compared with a propellant which is in the form of granules, having a much larger surface-area-to-mass ratio. Of course, the granules will be exhausted sooner than the block.
As a result of the problems with control of solid-fuel thrusters, later rockets, notably Goddard's and the German V-2 rockets, used fluid propellants, which could be valved to control thrust, and which did not suffer from changes in the configuration of the reacting material as with solid propellants. The fluid reactants, such as fuel and oxidizer, however, suffered from problems with control of the injection, so that too much of one or the other component might be injected at any moment, resulting in a tendency to explode if too much reactant was introduced into the combustion chamber, or to extinguish if too little was introduced. Either explosion or ignition extinction of the thruster could result in complete failure of a mission.
Liquid-oxidizer solid-fuel hybrid thrusters were devised in order to improve reliability. Such thrusters include a housing in which a solid fuel is located, and a tank of liquid oxidizer such as liquid oxygen (LO.sub.2), with a control system for allowing oxidizer to flow from the tank to the solid-fuel housing. A modern solid fuel is Hydroxyl Terminated Polybutadiene (HTPB), a solid synthetic elastomer, which generates combustible vapor when heated. The mechanism by which solid HTPB becomes vapor is not well understood, but may include sublimation, or decomposition of the high molecular weight HTPB into smaller gaseous molecules; it may also involve a melt layer. However the combustible vapor form of the HTPB is formed, it reacts with gaseous oxygen to form the reaction products which provide the rocket thrust.
When substantial amounts of thrust are desired, as for launch of large vehicles or payloads, large amounts of reactant materials are required in order to produce mass flow of reaction products sufficient to provide the desired lift. These large mass flows result in a need for one or more pumps for pumping liquid oxygen from the liquid oxygen storage tank to the solid fuel housing. A large booster or rocket may require a mass flow of liquid oxygen of more than one ton per second during operation. During operation, the solid fuel housing in which the oxidizer reacts with the solid fuel may be at a considerable pressure, possibly in the range of 1000 pounds per square inch (PSI). The liquid oxygen pump must pump the oxidizer mass against the housing pressure. Pumping a ton or more of mass per second against a back pressure of 1000 PSI clearly requires a large amount of pump drive power. This power may be provided from the launch pad until the engine is operating, whereupon some reaction products may be tapped off during, and after, liftoff, to continue operation of the pump. Alternatively, the power may be provided by an auxiliary engine.
If an auxiliary engine is used to operate the pump in a hybrid thruster, it must be possible to turn the auxiliary engine on reliably, since the main thruster operates only when the auxiliary engine drives the pump to cause a flow of oxidizer to the solid fuel. A liquid fueled auxiliary engine might be used, but the presence of the liquid fuel itself reduces the reliability of the vehicle as a whole. An improved liquid-oxidizer solid-fuel hybrid rocket thruster is desired.